This invention relates to a power converter system which comprises a plurality of rectifier circuits each incorporated with controlled rectifiers.
The power converter system of this type is suitable for use especially in AC electric rolling-stock, and an explanation will be given herein with respect to AC electric rolling-stock, by way of an example.
In an AC electric rolling-stock of one type, AC power is converted into DC power to drive a DC motor, by means of a power converter system utilizing controlled rectifiers such as thyristors which facilitate a noncontact control of the main circuit.
In this case, the power converter system is usually constituted with a rectifier circuit including either a bridge connection of thyristors or that of thyristors and diodes in combination. With this arrangement, the phase control for thyristors will cause harmonic components on the AC input current to the power converter system. The harmonic component is transmitted through a power line for the electric rolling-stock, thereby causing inductive interference in communication lines and the like around the power line.
In order to reduce such harmonic components, in place of a power converter system incorporated with a single rectifier circuit, a conventionally proposed power converter system is provided with a plurality of rectifier circuits (for example three to four) having DC terminals connected in series with each other.
In the proposal, a design is taken such that a specified rectifier circuit sequentially and cyclically controls its DC output voltage and remaining rectifier circuits control their DC output voltages in an on-off fashion.
However, in the case where the power converter system is constituted with a plurality of rectifier circuits, there occurs a problem that the DC output current (flowing through a DC motor) is caused to overshoot since when the specified rectifier circuit for sequentially controlling the output voltage reaches the ultimate value, the output voltage is commutated to the rectifier circuits for on-off control. The reason for this will be given as follows. Generally speaking, a power converter system is connected to a power line through a transformer. Accordingly, the transformer is provided with a secondary winding which is divided into a plurality of sets of coils each connected to AC terminals of respective rectifier circuits of the power converter system. On the other hand, in order to eliminate the overshot current caused when the output voltage is commutated, it is necessary to make equal completely the voltage regulation of the AC input voltage applied to respective rectifier circuits before and after the voltage commutation. Namely, the percentage commutating reactances of respective coils of the transformer secondary winding are needed to be made equal completely, where the percentage commutating reactance is defined by the following formula: ##EQU1##
Practically, however, the production of an ideal transformer in which the percentage commutating reactances of the secondary coils are made equal completely is impossible since the percentage commutating reactance inevitably suffers from error due to errors in production of the number of coils and magnetic coupling between the coils and the core of the transformer. As a result, the percentage commutating reactances of the secondary coils become irregular. Thus, if the percentage commutating reactance of the secondary coil of the rectifier circuit for sequential control exceeds that of the rectifier circuit for on-off control, the overshot current will result.
The overshot current thus caused is responsible for such adverse affect as:
(1) Creation of a surge current flowing through a DC motor which damages the motor;
(2) Thermal destruction of thyristors, diodes and the like elements constituting the power converter system;
(3) Increase in harmonic components on AC input current to the power converter system, causing the problem of inductive interferences;
(4) Rapid increase in driving torque of the DC motor, accompanied by slip of wheels of the electric rolling-stock; and
(5) Disturbance of a comfortable ride.